California citrus industry braces for impact of Asian citrus psyllid

California’s $1.3 billion commercial citrus industry is bracing for what could be a massive threat to the future of the state’s 290,000 acres of citrus trees which grow about 85 percent of the nation’s fresh citrus.

County, state, federal, and industry officials hope quick prevention efforts will impede the spread of the Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, found in backyards in San Diego County in early September.

ACP is a primary carrier of the deadly bacterial tree disease Huanglongbing (HLB) or citrus greening disease. The disease kills every tree it infects. It’s so devastating that Florida citrus growers have removed more than 60,000 acres of trees due to the ACP-spread HLB.

By mid-September more than 250 Asian citrus psyllids (ACP) were detected in San Diego’s South Bay Terrace area, plus a single psyllid in the community of Dulzura. Initial tests on early psyllid findings were negative for HLB.

“Huanglongbing (HLB) disease could potentially wipe out the California citrus industry for a period of time. I’m more terrified of this disease than anything else I’ve had to deal with as an entomologist,” stated Beth Grafton-Cardwell, University of California (UC), Riverside. “It is highly likely that somewhere in California citrus HLB is infecting a tree and when the ACP carrier spreads throughout the state it will pick it up and the disease will be off and running like in Florida.”

The detections prompted the California Department of Agriculture (CDFA) and the U.S. Department of Agriculture to place a quarantine on a portion of southern San Diego County to regulate the movement of citrus and closely-related plants and protect California’s commercial citrus-growing regions from the pest. CDFA said the county’s major citrus-producing region lies to the north of the boundary and is not included in the Sept. 11 quarantine.

“We are taking this preventive measure to stop the spread of this pest and protect our citrus crops,” said CDFA Secretary A.G. Kawamura. “This pest can carry a very serious disease that has the potential to cripple citrus plants beyond repair, so we are moving swiftly with this preventive measure to quarantine the area where the pest was found.”

If the pest and disease become established in California, a CDFA analysis based on Florida’s experience suggests it could cost (the California citrus industry) about $224 million annually, 20 percent of total citrus production.

About 80 percent of California’s citrus production is located in the San Joaquin Valley.

Ornamental citrus including orange jasmine and mock orange provide a home for ACP and may host the disease, but not show symptoms.

The ACP is a plant sucking, aphid-like insect that feeds on the leaves and stems of citrus trees and other citrus-like plants. Adult psyllids can live for one to two months. While the direct pest damage can be managed, the ultimate fear is the insect vectoring HLB. HLB is possibly the most devastating citrus disease in the world. HLB is also spread by grafting infected plant tissue.

San Diego County-based Citrus Advisor Gary Bender, University of California Cooperative Extension, says symptoms include: mottling of the leaves that resembles zinc deficiency; yellowing of the shoots; stunted trees that are sparsely defoliated and may bloom off-season; lopsided hard fruit with dark seeds; greening of the fruit; and a bitter juice comparable to turpentine or gasoline.

Essentially the tree must be removed so it doesn’t serve as an infection point for the rest of the grove. Infected trees typically die in three to five years.

Unlike the citrus tristeza virus where fruit from an infected tree can still be marketed, Grafton-Cardwell says fruit from HLB-diseased trees is absolutely worthless. “You can’t even juice it.”

No cure currently exists for HLB, Grafton-Cardwell says. “We hope researchers develop silver bullets quickly. Researchers are trying to determine the genome of the disease to determine how HLB interacts with the plant, learn how to block the disease, and then develop plants with resistance or tolerance. No disease-resistant citrus varieties exist at the moment,” she said.

HLB originated in Asia. The introduction into Florida was likely from someone who carried plant material from Asia to Florida for planting in a backyard.

Early detection of a diseased tree is difficult since the tree appears normal for a year or two after infection. HLB means “yellow shoot disease” in Chinese.

The ACP favors tender flush, explains Bob Blakely, director of grower services for California Citrus Mutual, Exeter, Calif. Since nursery plants flush all the time, it’s an ideal environment for the pest to thrive.

“Our biggest concern is HLB can lay dormant in a host plant for years; all it needs is a psyllid to feed on the infected plant whether it’s a citrus tree or an ornamental,” Blakely said. “Our biggest concern is ornamentals; there are a lot of ornamentals that are carriers of HLB. A heavy focus is on placing traps, sweeping with nets, and visual inspections of nursery operations.”

ACP was first detected in Florida in 1998 and quickly spread across Florida’s citrus belt in three years. By 2005, the psyllid had spread HLB to commercial citrus. All 32-citrus-producing counties now have ACP and HLB. Louisiana also has the pest and disease.

Most Florida citrus is used for concentrate and juice.

The pest, but not the disease, is in Texas, Alabama, Georgia, South Carolina, and Mississippi. Until last month’s ACP detection in San Diego — California and Arizona were the only remaining citrus-producing states in the nation without ACP or HLB.

HLB first caused devastation in Asia, India, parts of the Middle East, as well as South and Central America.

“We are very concerned that ACP has been found in Southern California,” Blakely said, “though we’re not surprised by the news since the pest was found earlier in Tijuana, Mexico. “It was never a matter of if we would find it (in California); it was a matter of when we would find it,” Blakely says. “The next step is to contain it and hopefully eradicate it. It is a very prolific pest and will present some big challenges.”

At press time, efforts were underway to gain emergency use of two chemical products for backyard use against ACP: the foliar-applied product Tempo SC Ultra for knockdown control and the soil-applied product Merit 2F for extended control, Grafton-Cardwell said.

Once an ACP picks up the HLB bacteria, it remains in the insect’s body for life. ACP can potentially spread the disease to every tree it reaches. “It doesn’t take a lot of psyllids or disease to cause an epidemic of the disease,” said Grafton-Cardwell, director of the Lindcove Research and Extension Center. “That’s what’s going on in Florida now.”

Citrus trees and fruit are a $61 million industry in San Diego County, according to the San Diego County Agriculture Commissioner’s Web site. About 170,000 tons of citrus were harvested in 2006 on 12,500 acres valued at about $34 million. Citrus varieties include Valencia and Navel oranges, lemons, kumquats, grapefruit, limes, and tangerines.

Bender fears the pest and the disease. “If these insects spread and they’ve had no luck in Florida controlling the spread, we could very well have a similar problem. If they hook up with the disease and get into our citrus production areas, it’s very serious. All growers can do is pull the trees out.”

HLB could be the “straw that breaks the camel’s back” financially for some San Diego County citrus growers. “If growers have to pay for a spray program, I think financially they’re done,” Bender said. “Some small growers are living on the edge financially and could throw in the towel.”

Educating homeowners about ACP and HLB is the major focus for Grafton-Cardwell and Bender. Grafton-Cardwell serves on a HLB task force led by the California Citrus Research Board and CDFA. She’s also part of a national group that’s accumulating and evaluating research on the disease.

“At this point our main focus is working with homeowners rather than commercial growers to educate them to look for the psyllid in yards and voluntarily treat it to slow the spread of the pest,” Grafton-Cardwell said.

Grafton-Cardwell and Georgios Vidalakis, director, citrus clonal protection program, UC Riverside, developed several steps to assist the commercial citrus industry to help keep ACP and HLB out of citrus orchards:

• When propagating citrus, use only budwood from a registered and tested source tree that is known to be free of disease.

• Purchase new rutaceous plants (citrus and closely-related plants) only from reputable nurseries.

• Do not bring the following into California: citrus or other rutaceous plants (especially Murraya and Bergera species) from Florida, Louisiana, Texas, Hawaii, Mexico or wherever the ACP and/or HLB are present.

• Monitor citrus and rutaceous plants for ACP. Look for adults, eggs, nymphs, and the white tubules that the nymphs produce.

• Monitor citrus for HLB symptoms: look for asymmetrical yellowing of leaves and small misshapen fruit.